1. Field of the Invention
The present invention relates in general to light water nuclear reactor designs which employ thorium as a fuel. The reactors can burn with the thorium, nonproliferative enriched uranium, weapons grade plutonium or reactor grade plutonium.
2. Description of the Prior Art
Nuclear power remains an important energy resource throughout the world today. Many countries without sufficient indigenous fossil fuel resources rely heavily on nuclear power for the production of electricity. For many other countries, nuclear energy is used as a competitive electricity producer that also diversifies their energy mix. Further, nuclear power also makes a very important contribution to the goals of controlling fossil fuel pollution (e.g., acid rain, global warming), and conservation of fossil fuels for future generations. In terms of numbers, nuclear power provides approximately 11% of the world's electricity. At the end of 1994, there were 424 nuclear power plants in 37 countries. Plants under construction will bring this number to approximately 500 plants by the end of the decade.
Although safety is certainly a major concern in the design and operation of nuclear reactors, another major concern is the threat of proliferation of materials which could be used in nuclear weapons. This is of particular concern in countries with unstable governments whose possession of nuclear weapons could pose a significant threat to world security. Nuclear power must therefore be designed and used in a manner which does not cause proliferation of nuclear weapons, and the resulting risk of their use.
Unfortunately, all present nuclear power reactors create large amounts of what is known as reactor grade plutonium For example, a typical 1,000 MWe reactor creates on the order of 200-300 kg per year of reactor grade plutonium. It is not difficult to reprocess this discharged reactor grade plutonium into weapons grade plutonium, and only approximately 7.5 kg of reactor grade plutonium is required to manufacture a single nuclear weapon. Accordingly, the fuel discharged from the cores of conventional reactors is highly proliferative, and safeguards are required to insure that the discharged fuel is not acquired by unauthorized individuals. A similar security problem exists with the vast stockpiles of weapons grade plutonium which have been created as the U.S. and the countries of the former U.S.S.R. have dismantled their nuclear weapons.
Other problems involved with the operation of conventional nuclear reactors concern permanent disposal of long term radioactive waste products, as well as the quickly diminishing worldwide supply of natural uranium ore. Regarding the former, government owned repository spaces are virtually nonexistent and the Yucca Flats project located in the United States has now been delayed by Congress. As to the latter, significant problems with supplies of natural uranium ore are foreseen within the next 50 years.
As a result of the foregoing problems, attempts have been made in the past to construct nuclear reactors which operate on relatively small amounts of nonproliferative enriched uranium (enriched uranium having a U-235 content of 20% or less), and do not generate substantial amounts of proliferative materials, such as plutonium. Examples of such reactors are disclosed in my two previous international applications, Nos. PCT/US84/01670, published on Apr. 25, 1985 under International Publication No. WO 85/01826, and PCT/US93/01037, published on Aug. 19, 1993 under International Publication No. WO 93/06477. The '826 and '477 applications both disclose seed-blanket reactors which derive a substantial percentage of their power from thorium fueled blankets. The blankets surround an annular seed section which contains fuel rods of nonproliferative enriched uranium. The uranium in the seed fuel rods releases neutrons which are captured by the thorium in the blankets, thereby creating fissionable U-233 which burns in place, and generates heat for powering the reactor.
The use of thorium as a nuclear reactor fuel in the foregoing manner is attractive because thorium is considerably more abundant in the world than is uranium. In addition, both of the reactors disclosed in the '826 and '477 applications claimed to be nonproliferative in the sense that neither the initial fuel loading, nor the fuel discharged at the end of each fuel cycle, is suitable for use in the manufacture of nuclear weapons. This is accomplished by employing only nonproliferative enriched uranium as the seed fuel, selecting moderator/fuel volume ratios which minimize plutonium production and adding a small amount of nonproliferative enriched uranium to the blanket whose U-238 component uniformly mixes with the residual U-233 at the end of the blanket cycle, and "denatures" the U-233, thereby rendering it useless for manufacture of nuclear weapons.
Unfortunately, Applicant has discovered through continued research that neither of the reactor designs disclosed in the aforementioned international applications is truly nonproliferative. In particular, it has now been discovered that both of these designs result in a higher than minimum production of proliferative plutonium in the seed due to the annular seed arrangement. The use of the annular seed with both an inner, central blanket section and an outer, surrounding blanket section cannot be made nonproliferative because the thin, annular seed has a correspondingly small "optical thickness" which causes the seed spectrum to be dominated by the much harder spectrum of the inner and outer blanket sections. This results in a greater fraction of epithermal neutrons and a higher than minimum production of proliferative plutonium in the seed.
Both of these previous reactor designs are also not optimized from an operational parameter standpoint. For example, moderator/fuel volume ratios in the seed and blanket regions are particularly crucial to minimize plutonium production in the seed, permit adequate heat removal from the seed fuel rods and insure optimum conversion of thorium to U-233 in the blanket. Further research indicates that the preferred moderator/fuel ratios disclosed in these international applications were too high in the seed regions and too low in the blanket regions.
The previous reactor core designs were also not particularly efficient at consuming the nonproliferative enriched uranium in the seed fuel elements. As a result, the fuel rods discharged at the end of each seed fuel cycle contained so much residual uranium that they needed to be reprocessed for reuse in another reactor core.
The reactor disclosed in the '477 application also requires a complex mechanical reactor control arrangement which makes it unsuitable for retrofitting into a conventional reactor core. Similarly, the reactor disclosed in the '826 application cannot be easily retrofitted into a conventional core either because its design parameters are not compatible with the parameters of a conventional core.
Finally, both of the previous reactor designs were designed specifically to burn nonproliferative enriched uranium with the thorium, and are not suitable for consuming large amounts of plutonium. Thus, neither of these designs provides a solution to the stockpiled plutonium problem.